1. Field of the Invention
This invention relates to an apparatus and process for the vaporization of liquid precursors and the controlled delivery of those precursors to form films on suitable substrates. More particularly, this invention relates to an apparatus and a method for the deposition of a high dielectric constant film, such as Tantalum Oxide (Ta2O5), on a silicon wafer to make integrated circuits useful in the manufacture of advanced dynamic random access memory (DRAM) modules and other semiconductor devices.
2. Background of the Invention
The desire for greater capacity integrated circuits (ICs) on smaller sized devices has increased interest in replacing today""s 64 megabit DRAM with memory devices in the range of 256 megabit, 1 gigabit and higher. This need for increased capacity on the same or smaller substrate footprint device makes it necessary to replace conventional dielectric films previously used in stacked capacitor formation, such as silicon dioxide (SiO2), with dielectric films having higher dielectric constants. Capacitors containing high-dielectric constant materials, such as Ta2O5, usually have much larger capacitance densities than standard SiO2xe2x80x94Si3N4xe2x80x94SiO2 stack capacitors making them the materials of choice in IC fabrication. High dielectric constant films are desirable because they provide higher capacitance which enables closer spacing of devices without electrical interference which can increase transistor density. One material of increasing interest for stack capacitor fabrication is Tantalum Oxide which has a relative dielectric constant more than six times that of SiO2.
One common method of forming Tantalum oxide film is to vaporize a liquid Tantalum precursor and then deliver the Tantalum vapor to a deposition chamber. Such vapor delivery methods face numerous challenges because of the low vapor pressure of typical Tantalum precursors such as (Ta(OC2H5)5) or TAETO and Tantalum Tetraethoxide Dimethylaminoethoxide (Ta(OEt)4(OCH2CH2N(Me)2) or TAT-DMAE, both of which are liquid at room temperature and pressure. FIG. 1 graphically illustrates the large variation between the vapor pressure of Tantalum precursors and other representative prior-art precursors for other semiconductor related processes. For example, at 100xc2x0 C. and 1 atm TAT-DMAE has about 0.3 Torr vapor pressure while TAETO has about 0.03 Torr vapor pressure. The vapor pressures for Tantalum precursors are remarkably lower than those precursors typically used in prior art vapor delivery systems which are intended to vaporize precursors having much higher vapor pressures. Again referring to FIG. 1, at 100xc2x0 C. and 1 atm, TEOS, (Tetra Ethyl Ortho-Silicate) which is commonly used in chemical vapor deposition processes to form SiO2 films and is the subject of several prior art vapor delivery systems, has a vapor pressure of almost 100 Torr. As a result of this vast difference in vapor pressure, prior art vapor delivery systems did not encounter nor provide solutions to many of the challenges resulting from the use of very low vapor pressure precursors such as TAETO and TAT-DMAE.
Prior art vapor delivery systems commonly involved the use of an integrated liquid flow controller and vaporizer without a positive liquid shut-off valve. Such a configuration, when used with low vapor pressure Tantalum precursors, can lead to problems stabilizing the Tantalum vapor output and difficulty achieving the constant, repeatable Tantalum vapor output desirous in semiconductor device fabrication. Previous delivery systems, based upon experience with TEOS and other relatively high vapor pressure materials, allow for the flow controller and vaporizer to be separated by considerable distance or attach no significance to the distance between vaporizer and liquid flow meter. Positioning the vaporizer and flow meter according to prior art systems fail to adequately control Tantalum precursor vapor. Previous delivery systems are intended for use with higher vapor pressure precursors whose residuals can be adequately removed by applying low pressure or xe2x80x9cpumping-downxe2x80x9dthe lines while flowing an inert gas like nitrogen. Purging techniques such as these fail with Tantalum systems because the low vapor pressure residual tantalum vapor creates a need to introduce a solvent, such as isopropyl alcohol, ethanol, hexane, or methanol into both the vaporization system and supply lines to remove residual Tantalum precursor vapor.
Previous vapor delivery systems avoided precursor vapor condensation by heating the delivery lines usually by resorting to a flexible resistive heater which is wrapped around and held in direct contact with the line, and then insulated. Since such systems typically operated with precursor materials having a wide temperature range within which the precursor remains vaporous, the requirement to sample the temperature of any section of the heated line was low and typically a single thermocouple would be used to represent the temperature of piping sections as long as four to six feet. Since the object of large scale temperature control systems, such as wrapped lines and jacket-type heaters used in prior art systems, is to heat and monitor an average temperature of a large section of piping, such systems lack the ability to specifically control a single, smaller section of the vapor piping and generally have very low efficiency when higher line temperatures are desired. Vaporized Tantalum delivery systems maintain the Tantalum vapor above the vaporization temperature but below the decomposition temperature for a given Tantalum precursor. Once formed, the vaporous Tantalum must be maintained at elevated temperatures between about 130xc2x0 C. and 190xc2x0 C. for TAT-DMAE and between about 150xc2x0 C. and 220xc2x0 C. for TAETO. Because of the relatively high temperatures needed and the narrow temperature band available to low vapor pressure precursors such as TAT-DMAE and TAETO, Tantalum and other low vapor pressure liquid delivery systems would benefit from vapor delivery line temperature controls and methods which can achieve and efficiently provide the higher temperatures and greater temperature control needed for Tantalum vapor delivery. Additionally, finer temperature controls are desirous since the useable temperature range of vaporized low pressure liquids is smaller than prior art liquids. Because higher temperature vapor delivery is needed, Tantalum delivery systems would benefit from designs which minimize the length of heated vapor delivery lines. Minimizing the length of lines requiring heating not only reduces the overall system complexity but also decreases the footprint or overall size of the system.
Current methods of Tantalum Oxide deposition use reaction rate limited chemical vapor deposition techniques. In reaction rate limited deposition processes, the deposition rate achieved under these conditions is largely influenced by the temperature of the reaction environment. Existing chemical vapor deposition reactors do not sufficiently address the thermal losses between the substrate onto which the Tantalum film is to be formed and internal chamber components such as the gas distribution showerhead. Such thermal losses and the resultant non-uniform thickness of deposited Tantalum illustrate the barriers to commercially viable Tantalum oxide film formation techniques. However, with commercially viable Tantalum deposition rates also comes the need for a viable, insitu cleaning process which can remove Tantalum deposition formed on internal chamber components without harm to these components.
There is a need for a Tantalum deposition apparatus which can deliver vaporized, measured Tantalum precursors which have been adequately mixed with process gases to a reaction chamber which provides a controlled deposition environment which overcomes the shortcoming of the previous systems. Additionally, there is also a need for a deposition apparatus capable of in-situ cleaning.
In one aspect of the present invention, a deposition apparatus is provided for depositing tantalum oxides and other materials especially those with low vapor pressure liquid precursors which are provided as liquid to a vaporizer to be converted into the vapor phase. The vapor is then transported from the vaporizer into a substrate processing region via temperature controlled conduits where the temperature within the conduits allows neither condensation nor decomposition of the vaporized precursor. Separate thermocouple, heater, controller units control the temperature conduits so as to maintain a temperature within the conduit above the condensation temperature but below the decomposition temperature of a given precursor vapor or, more particularly, between about 130xc2x0 C. and 190xc2x0 C. for a Tantalum precursor such as TAT-DMAE or between about 150xc2x0 C. and 220xc2x0 C. for a Tantalum precursor such as TAETO. Additionally, the temperature controlled conduits could provide a temperature gradient along the vapor flow path between the vaporizer and the processing region. Other precursor source materials and dopants, alone or in combination, are also contemplated.
In another aspect of the present invention, a resistive heater is embedded in the lid of the processing chamber which provides for elevated temperatures within the gas box formed between the lid and the showerhead gas distribution plate.
In another aspect of the showerhead gas distribution plate of the present invention, the specific shape and spacing of the apertures which allow gas to enter into the processing region of the processing chamber present an angled lower surface towards a substrate within the processing region. The spacing and specific shape of the apertures allow more incident energy from the substrate to be absorbed into instead of reflected off the showerhead or where the emissivity of the showerhead is increased by the angled lower surface. Another feature of the present invention is modifying the surface of the showerhead lower surface which faces a substrate in the processing region. The modification results in a surface which has a high emissivity relative to the emissivity changes which result from film accumulation on the surface of the showerhead as well as other factors. Each of these features alone or in combination helps minimize substrate heat losses which contribute to temperature nonconformities. The net effect of the aperture hole shapes, spacing and high emissivity modification or coating is that most of the radiation emitted from the substrate surface is absorbed by the showerhead.
In another aspect of the present invention, a deposition system is provided for depositing tantalum oxides and other materials, especially those with low vapor pressures alone or in combination with a variety of processing gases or dopants. The deposition system is comprised of a heated exhaust system, a liquid delivery system, a remote plasma generator, and a processing chamber. In operation, the deposition system provides a method and apparatus capable of the controlled delivery of a variety of vaporized, low vapor pressure liquid precursors and activated species into a substrate processing region for cleaning, deposition or other operations.